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Trefoil factor family 2 (TFF2), also known as spasmolytic polypeptide, is a member of the trefoil family of peptides and is expressed primarily in the mucous neck cells of the gastric mucosa. To study the physiologic role of TFF2, we have generated TFF2-deficient mice through targeted gene disruption. Homozygous mutant mice were viable and fertile without obvious gastrointestinal abnormalities. However, quantitative measurements revealed a significant decrease in gastric mucosal thickness and in gastric mucosal proliferation rates. In addition, there was a twofold increase in activated parietal cells resulting in a twofold increase in basal and stimulated gastric acid output and an undetectable serum gastrin level. The TFF2-deficient mice also showed a significant increase in the degree of gastric ulceration after administration of indomethacin. Taken together, these results suggest a physiologic role for TFF2 to promote mucosal healing through the stimulation of proliferation and downregulation of gastric acid secretion.

Mutations in the adenomatous polyposis coli (APC) tumor suppressor gene occur in most colorectal cancers and lead to activation of beta-catenin. Whereas several downstream targets of beta-catenin have been identified (c-myc, cyclin D1, PPARdelta), the precise functional significance of many of these targets has not been examined directly using genetic approaches. Previous studies have shown that the gene encoding the hormone gastrin is activated during colon cancer progression and the less-processed forms of gastrin are important colonic trophic factors. We show here that the gastrin gene is a downstream target of the beta-catenin/TCF-4 signaling pathway and that cotransfection of a constitutively active beta-catenin expression construct causes a threefold increase in gastrin promoter activity. APC(min-/+) mice overexpressing one of the alternatively processed forms of gastrin, glycine-extended gastrin, show a significant increase in polyp number. Gastrin-deficient APC(min-/+) mice, conversely, showed a marked decrease in polyp number and a significantly decreased polyp proliferation rate. Activation of gastrin by beta-catenin may therefore represent an early event in colorectal tumorigenesis and may contribute significantly toward neoplastic progression. The identification of gastrin as a functionally relevant downstream target of the beta-catenin signaling pathway provides a new target for therapeutic modalities in the treatment of colorectal cancer.

Gastrin is a peptide hormone involved in the growth of both normal and malignant gastrointestinal tissue. Recent studies suggest that the glycine-extended biosynthetic intermediates mediate many of these trophic effects, but the in vivo relevance of glycine-extended gastrin (G-Gly) has not been tested. We have generated mice (MTI/G-GLY) that overexpress progastrin truncated at glycine-72 to evaluate the trophic effects of G-Gly in an in vivo model. MTI/G-GLY mice have elevated serum and colonic mucosal levels of G-Gly compared with wild-type mice. MTI/G-GLY mice had a 43% increase in colonic mucosal thickness and a 41% increase in the percentage of goblet cells per crypt. MTI/G-GLY mice exhibited increased colonic proliferation compared with wild-type controls, with an expansion of the proliferative zone into the upper third of the colonic crypts. Continuous infusion of G-Gly into gastrin-deficient mice for two weeks also resulted in elevated G-Gly levels, a 10% increase in colonic mucosal thickness, and an 81% increase in colonic proliferation when compared with gastrin-deficient mice that received saline alone. To our knowledge, these studies demonstrate for the first time that G-Gly's contribute to colonic mucosal proliferation in vivo.

Incompletely processed gastrins have been postulated to play a role in growth of the gastrointestinal tract, but few studies have examined the effects of progastrin on mucosal proliferation in vivo. Human gastrin gene expression and progastrin processing were therefore studied in transgenic mice containing a human gastrin (hGAS) minigene, and compared to processing in mice bearing an insulin gastrin (INS-GAS) transgene that overexpresses amidated gastrin. Progastrin processing was studied using region-specific antisera and radioimmunoassays, biosynthetic labeling, immunoprecipitation, and HPLC. Proliferative effects due to overexpression of processed and unprocessed gastrin in INS-GAS and hGAS mice, respectively, were determined using routine histology and BrdU incorporation. The pancreatic islets of INS-GAS mice were able to produce carboxyamidated G-17, resulting in a twofold elevation of serum amidated gastrin, marked thickening of the oxyntic mucosa, and an increased BrdU labeling index (LI) of the gastric body. In contrast, livers of adult hGAS mice expressed abundant human gastrin mRNA and human progastrin but were unable to process this peptide to the mature amidated form, resulting in markedly elevated serum progastrin levels and normal amidated gastrin levels. Nevertheless, there was a marked increase in the BrdU labeling index of the colon in hGAS mice (LI 7.46+/-1.90%), as well as in INS-GAS mice (LI 6.16+/-1.17%), compared to age-matched, wild type control mice (LI 4.01+/-0.98%, P < 0.05). These studies suggest that incompletely processed gastrin precursors may contribute to colonic mucosal proliferation in vivo.